1. Field of the Invention
The field of the invention is that of optical guides used for transporting a collimated image. These guides may notably be used for the production of ocular vision optical systems. These systems make it possible to overlay an image output by a micro-display on the outside landscape. Its applications relate to the mass-market sector as well as the field of aeronautics.
2. Description of the Prior Art
A display system comprising such a guide is schematically shown in FIG. 1. It comprises a small imager 1, collimating optics 2 and the optical guide 10. The function of the optical guide is to convey the collimated image to the eye of the user while creating the minimum of visual disturbances and while being as discrete, small and light as possible. In its basic version, it comprises a plate 11 with planar and parallel faces of small thickness in which a semi-reflective plate 12 is inserted. This plate causes only a very slight disturbance of the vision of the outside. The light beams output by the displayer propagate by total reflection inside the optical guide, are reflected by the semi-reflective plate 12 and exit the optical guide towards the eye of the user O. They are symbolized by discrete arrows in FIG. 1 and the following figures.
If the optical guide 10 comprises only a single semi-reflective plate, the observer O can see only a narrow portion of the image, the pupil of the system being situated far from the eye and being by nature small in size. In order to palliate this drawback in a given dimension, the number of semi-reflective plates 12 is increased as indicated in FIG. 2, which shows a cut-away view of an optical guide 10 comprising a plurality of semi-reflective plates 12. These semi-reflective plates are all perfectly mutually parallel.
For certain applications, notably those requiring wide fields of vision, it is necessary to further enlarge the pupillary zone in both dimensions of the space. In this case, as illustrated in FIG. 3, which shows a perspective view of a guide giving such a pupil, the optical guide 10 is composed of two primary optical elements 20 and 30. The first element 20 is a first optical guide with planar and parallel faces, comprising several first semi-reflective plates 12 that are mutually parallel and make it possible to enlarge the pupil in a first given direction. The second element 30 is a second optical guide with planar and parallel faces, comprising several second semi-reflective plates 31 that are mutually parallel and make it possible to enlarge the pupil in a second direction perpendicular to the first direction. The two guides meet at a common face 22 and form a single plate with planar and parallel faces.
Such optical guides still have one drawback. Each parallel light beam representing a given field must be large enough to “cover” the entirety of the planar faces of the guide when successive total internal reflection on these same faces occurs, in such a way that the set of beams reflected by the successive semi-reflective plates totally covers a pupil zone without “holes”. However, if the guide only comprises semi-reflective plates, it is possible that this may not be the case. As may be seen in FIG. 4, which shows a cut-away view of a light guide 10 comprising three mutually parallel, inclined semi-reflective plates 12, although the light beams cover the whole entrance pupil of the guide the beams reflected by the various plates do not cover a homogenous zone.
To solve this problem, several technical solutions have been proposed. The patent FR 2-937 152 entitled “Dispositif de guidage optique d'un faisceau lumineux” (Device for optical guiding of a light beam) proposes to enlarge the injection section of the light beams arranged at the input of the optical guide. This solution makes it possible to enlarge the injection section in one dimension of space but is more difficult to apply to two dimensions of space. Patent application WO 2009/009 268 entitled “Substrate-guided relays for use with scanned beam image sources” proposes to introduce, into the interior of the waveguide, a plate that is planar, semi-reflective and parallel to the planar external faces of the guide, making it possible to double beams incident on this plate in a give direction. The reader should refer to FIGS. 3A, 3B and 3C of this application for details regarding propagation of the light beams inside the optical guide. Patent application EP 0 399 865 entitled “Dispositif optique pour I'introduction d'une image collimatée dans le champ visuel d'un observateur et casque comportant au moms un tel dispositif” (Optical device for the introduction of a collimated image into the field of vision of an observer and helmet comprising at least one such device) also proposes to insert a plate that is planar, semi-reflective and parallel to the planar external faces of the guide with the same aim. The reader should in particular refer to FIG. 1 of this patent. Finally, patent FR 2 662 821 entitled “Ensemble optique pour l'introduction, en surimpression, d'une image dans le champ visuel d'un observateur et casque comportant au moins un tel ensemble” (Optical assembly for the introduction, in superposition, of an image in the field of vision of an observer and helmet comprising at least one such assembly) is an improvement of the preceding application. The particularity of the light guide claimed is that it comprises both a plate that is planar, semi-reflective and parallel to the planar external faces of the guide and a concentration “block” formed by a stack of planar mirrors the surfaces of which are perpendicular to said planar external faces. The reader should refer to FIGS. 2 and 3 of this patent for geometrical details.
An image guide with internal semi-reflective plates is a complicated optical component to manufacture. It is composed of numerous processed and adhesively bonded optical elements which after assembly, must exhibit perfectly planar surfaces with extremely precise orientations. Optical guides of the prior art, to solve the problem of pupillary “holes”, add various components to the inside of the optical guide, rendering its manufacture complicated and expensive, and introducing a possible source of parasitic images.